The Mule sees Clint Eastwood reckon with his lone cowboy, political conservative persona

It's been a long and storied journey, but at the tender age of 88, Clint Eastwood has finally settled in as the wizened, reflective heart of American cinema — and made what might be one of its great road movies.

Eastwood's latest film, his 40th as director and his first leading role in a decade, reunites him with Gran Torino writer Nick Schenk, for a drawn-from-real-life story of yet another cantankerous Korean War veteran who's out of step with modern social mores.

But The Mule offers a more curious variation on that character type — a stubborn loner who seems less grouchy than mildly bewildered, his grizzled behaviour merely quaint rather than actively antagonistic.

Ninety-year-old Earl Stone is something of a lifelong drifter, estranged from his family — including ex-wife Mary (the ever-wonderful Dianne Wiest) and daughter Iris (Eastwood's real-life daughter, Alison) — and largely obsessed with cultivating his beloved daylilies.

In fact, he's a champion gardener.

"They bloom just one day, then that's the end of it," Earl says, sketching the film's broader life philosophy by way of his flowers.

If Clint Eastwood as an award-winning floriculturalist isn't enough for you (and frankly, there needs to be an entire other film dedicated to that storyline) then looming financial troubles soon see Earl accepting an unexpected offer to run drugs for the local branch of a Mexican cartel.

It's the perfect cover: who would suspect an old white man of smuggling kilos of cocaine cross-country?

Earl is soon zipping down the highway in his vintage Ford pick-up, and later a glistening black Lincoln truck, warbling the classic rock hits — like Spiral Staircase's More Than Yesterday — that will comprise many of the movie's more indelible moments.

Meanwhile, two DEA agents (played by Bradley Cooper and Michael Pena with the comic generosity of their own self-contained buddy-cop movie) are out to bust the cartel's highway smuggling operation.

It's the sort of potentially broad premise that requires assured hands and they don't come much steadier than Eastwood's.

In a typically restrained performance, the octogenarian star brings texture, melancholy and a wonderful sense of comedy to Earl, affecting a frail hunch, with that famous side-scowl transformed into a marker of sporadic confusion.

Whether engaging in a politically incorrect chat with a lesbian biker gang or performing a lively Dean Martin duet with the gangsters hired to shadow him, Earl is consistently warm and amusing company.

Integral to Eastwood's worldview, The Mule is also distrusting of institutions and suspicious of law enforcement and its racial profiling.

In one scene, a suspected Latino motorist is pulled over by the DEA purely on the basis of his looks, while the agents' fruitless search to uncover the mule — who's right under their nose — is similarly hampered by their pursuit of suspects that fit their class and racial profile.

Unlike the police, Earl is straight shooting with everyone he approaches — however flawed, and often rogue, his manner may be.

Admittedly, it's also indicative of a certain type of conservative's idealised self-image.

"You're so old you probably lost your filter," Cooper's agent tells Earl over a chance breakfast meeting.

"Really? I never realised I had one." Earl replies, in what feels like a cheeky riposte to Eastwood's political critics.

Eastwood plays these comedic exchanges with the lightest touch, yet the movie also wears the weight of seven decades' worth of personal and cinematic history — brought about by both sheer force of longevity and years of well-honed insight.

In another film, a highway needle-drop of Willie Nelson's On the Road Again might feel creatively bankrupt, but here it's like an acknowledgment of cowboy survival from one outlaw peer to another.

Even a sudden family tragedy, designed to push the story toward its dramatic resolution, is flipped from borderline cheese to genuinely affecting, thanks to the gravitas of Wiest and Eastwood.

"You're the love of my life and you're the pain of my life," she tells him, in a moment sure to leave only the most cynical eye dry.

And the film is very much concerned with Earl's reckoning with this personal accountability.

It's a complexity that Eastwood is willing to embrace, both as the character, filmmaker, and icon of such a sustained passage of American movie history.

Earl functions as a perceptive dissection — and in many cases, deflation — of the loner myth perpetuated by so many of Eastwood's famous movies, from the Spaghetti Westerns to his morally divisive Dirty Harry cycle.

At one point in the film, an unsuspecting highway patrolman even compliments Earl on his "great Jimmy Stewart" impression, drawing a surprising comparison between Hollywood's longest-standing tough guy and the 'aw, shucks' conscience of its golden age.

Thing is, for a movie that seamlessly mixes gentle humour, sentimentality and a wry sense of self-examination, the comparison doesn't feel like much of a reach.